Functional neuroimaging offers a unique and powerful method for relating genetic differences to individual variability at the level of the brain. For example, a recent fMRI study showed that healthy individuals differ in the amount of amygdala activation while viewing emotional faces, depending on their genotype for a polymorphism in the serotonin transporter (5HTT). Serotonin is an important neurotransmitter that has been implicated in the etiology of many psychiatric illnesses including autism. Despite evidence for serotonergic dysfunction, autism remains a poorly understood disorder that shows substantial heterogeneity among affected individuals. This proposal examines how functional polymorphisms in genes associated with the serotonin system, such as 5HTT and monoamine oxidase A (MAO-A), may influence individual differences in brain function in an autistic population. The specific aims are (1) to examine the effect of genetic differences in the serotonin system on amygdala response to facial expressions among healthy children, and (2) among autistic children using fMRI, and finally, (3) to assess potential genetic contributions to behavioral severity along the social symptom domain in autistic individuals.